vc4: Add support for sampling from sRGB.

vc4: Add support for sampling from sRGB.

This isn't perfect -- the filtering is happening on the srgb values, and
we're decoding afterwards, which is not what you want.  I think that's the
cause of some additional texwrap(GL_CLAMP, LINEAR) failures, though many
other texwrap tests on srgb start to pass since unfiltered values come out
correct.